Locked Rotor 51LR and Incomplete Sequence 48 Protection

What 51LR, 48 and 66 protect

When an induction motor starts, or when its shaft jams, the rotor is at standstill and the motor draws its locked rotor current, several times rated current. Under that condition almost all the energy becomes heat in the rotor bars, which get no ventilation because the motor is not turning. The manufacturer states this limit as the safe stall time (safe locked rotor time), cold and hot: the seconds the motor withstands at standstill at full voltage before it is damaged.

Three functions of the motor protection relay look after that limit:

The suffix “LR” is not a number from the C37.2 list: manufacturers and studies use it to tell this application apart from general 51 overcurrent. That is why the same relay may show these functions grouped together; ABB, for example, groups start supervision under “49, 66, 48, 51LR” in the REM630 documentation and calls another function, also labelled 51LR, stall (jam) protection.

How they work

Start supervision (48). The relay detects the start (current rises above a threshold, or the contactor closes) and starts timing. If the current has not dropped close to running current before the set time, it trips. Some relays integrate the starting current as I²t instead of using a fixed time, which accounts for the lower current and longer start at reduced voltage.

Locked rotor or jam (51LR). With the motor already running, a load that jams (a mill that chokes, a conveyor that gets stuck, a pump with a foreign object) drives the current up towards locked rotor current. The 51LR trips if that current lasts longer than the set time, always below the motor’s safe stall time. Because it is enabled only after the start, it can be much faster than the 49.

Starts limit (66). Each start leaves heat in the rotor. The 66 counts starts in a time window, or uses the thermal capacity calculated by the 49 model, and blocks a new start until the motor cools down. It is the most frequent reason for “the motor won’t let me start it” after several attempts.

Setting criteria

Settings are never copied from another motor. They depend on:

• The motor thermal limit curve and the cold and hot safe stall times (IEEE 620 describes how these curves are presented).
• The actual starting time, taken from the motor starting study or from the relay’s start records, at the lowest expected voltage.
• The starting method: a soft starter or an autotransformer starter lengthens the start and changes the shape of the current.
• The number of starts the process needs and the number the manufacturer allows.
• Coordination with thermal overload 49, 50/51 overcurrent and the upstream fuse or breaker.

The basic rule is that the trip must come before the safe locked rotor time and after the normal starting time, with margin for low voltage.

How each manufacturer implements it

These functions exist in every motor relay, but each brand organizes them its own way:

• ABB (Relion 615 and 630) groups start supervision with the start counter and keeps jam protection separate.
• SEL, GE Multilin, Siemens SIPROTEC and Schneider (MiCOM, Easergy) have prolonged start, locked rotor or jam, and starts limit elements, with different names, enabling logic and reset criteria, and some accept a speed switch input.

In practice, the same protection criterion becomes different settings and logic depending on the model and its software. That is where mistakes appear: a jam element enabled during the start, a prolonged start timer that does not see the soft starter ramp, or a start counter that does not match what the motor allows.

Why it trips or blocks

• Start longer than the setting: low voltage, more load or inertia, soft starter ramp.
• Copied setting from another motor, or from the data sheet of a motor of a different rating.
• Real jam of the mechanical load, or damaged bearings.
• Repeated starts that use up the thermal capacity: the 66 is right to block.
• Poorly defined start detection: the relay does not see the contactor close, or the current threshold is wrong.
• Undersized CTs that saturate with the starting current.

If the motor tripped and the cause is not clear, the relay’s start report, oscillography and event list will explain it. We analyze it in our relay trip analysis.

How it is tested

Secondary injection simulates a normal start and a prolonged one, a rotor that jams while running and a series of successive starts, and checks that each function operates within its time and that the enabling logic and restart blocking behave as designed. At commissioning, the first real start is compared with the setting.

Motor relay configuration

Did your motor trip on prolonged start or locked rotor with no apparent fault? Our team reviews the relay events and settings remotely, compares them with the thermal curve and the actual starting time, and loads the corrected settings. Tell us the brand and model through remote relay configuration or write to us on the contact page.

Related guides

• Thermal overload 49
• Negative sequence 46
• Motor differential 87M

References

• IEEE C37.2, IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.96, IEEE Guide for AC Motor Protection.
• IEEE 620, IEEE Guide for the Presentation of Thermal Limit Curves for Squirrel Cage Induction Machines.
• ABB, REM630 Motor Protection and Control, application manual.

Frequently asked questions